Fast Ejaculation

So I am 18 and me and my girl have been having sex for a while. At first it was good. We would do it with no condom and I would ejaculate but would keep on going for a while. Now I ejaculate fast and my penis goes down and that's that. I ejaculate really fast and now im scared she'll leave me because I can't please her. What do I do to fix this? Is there something wrong with me?

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